High temperature brazing alloy system

ABSTRACT

A high temperature, low vapor pressure, brazing alloy system is disclosed which is useful for assembling components of electron tubes. The brazing alloy is particularly useful in the assembly of refractory elements such as cathodes and heaters because degradation of emission caused by the metals comprising the brazing alloy is minimal.

United States Patent Bondley [54] HIGH TEMPERATURE BRAZING ALLOY SYSTEM[72] Inventor: Ralph J. Bondley, Scotia, N.Y. [73] Assignee: GeneralElectric Company 221 Filed: Jan. 18,1971

211 Appl.No.: 107,011

[52] U.S. Cl ..29/194, 29/198, 75/134 V [51] Int. Cl. ..B32b 15/02, C22c27/00 [58] Field of Search ..29/l98, 194; 75/l34 V [56] References CitedOTHER PUBLICATIONS Constitution of Binary Alloys, First Supplement,

[ 51 Sept. 19, 1972 Rodney P. Elliott, pages 859- 860, New York, Mc-Graw-Hill, 1965.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L. WeiseAttorney-Nathan .l. Cornfeld, .lohn P. Taylor, Frank L. Neuhauser, OscarB. Waddell and Joseph B. Forman 5 7] ABSTRACT 4 Claims, 1 Drawing FigurePATENTEUSEP 19 I972 INVENTORZ RALPH J. BONDLEY HIS ATTOR HIGHTEMPERATURE BRAZING ALLOY SYSTEM The invention herein described was madein the course of or under a contract or subcontract thereunder with theDepartment of the Army.

BACKGROUND OF THE INVENTION This invention relates to high temperaturebrazing alloys. More particularly, the invention relates to brazingmaterial which is useful for joining components of electron tubes suchas refractory matrix cathodes which operate in a high temperature rangeof over l,000C.

Many vacuum devices, such as electron tubes and various components orsubassemblies of electron tubes are conveniently fabricated by brazingtechniques. Various metals and alloys are used for this purpose. Theparticular properties of the brazing alloy desired are determined by theenvironment and end usage. For example, if the brazing alloy is used inthe fabrication of a device such as an electron tube which will beoperated at elevated temperatures in vacuum, the vapor pressure of thealloy becomes a critical property. If the alloy is used in thefabrication of a cathode, it must not adversely effect the thennionicemission of the cathode by coating the emitting surface of the brazingalloy or otherwise contaminating it by metal migration, diffusion, oralloying. The problem is particularly acute in the case of refractorymatrix cathodes operating in the range of l,000 to l,400C, as there arevery few known metals or alloys having melting points above thesetemperatures which also meet the vapor pressure criteria.

Nickel-molybdenum alloys, having a melting point above 1,305C have oftenbeen used as a brazing alloy for this type of application. However, theuse of this alloy in fabricating barium-strontium-tungstate matrixcathodes invariably results in reduced cathode activity. Another alloywith a melting point in the 1,400C range which has been used is composedof thorium and hafnium. The deterioration in cathode performance whenusing this alloy as a brazing material is less severe. However, thealloy is difficult to fabricate and use, since it can only be handled invacuum or inert atmospheres not in a reducing atmosphere. The flowcharacteristics of this active alloy are also difficult to control orpredict.

SUMMARY OF THE INVENTION drawings and description.

In accordance with the invention a brazing system comprising a highmelting point alloy of tungsten and vanadium is provided. The alloycomprises a composition of about 4.5 atomic percent tungsten weightpercent) and about 95.5 atomic percent vanadium (85 weight percent)having a melting point of about 1,635C.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section of a cathodeand heater assembly bonded together using the brazing alloy of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As previously described above,the brazing alloy of the invention comprises a Tungsten-Vanadium alloy.The composition of the alloy is about 4.5 atomic percent or 15 weightpercent tungsten and about 95.5 atomic percent or weight percentvanadium representing a minimum melting point or eutectic mixturemelting at about 1,635C.

The alloy can be conveniently prepared in situ by previously mixingpowdered tungsten and powdered vanadium in the proper weightproportions, preparing a slurry of this mixture by mixing the powderedmixture with a fugitive liquid such as'water, ethyl alcohol, and amylacetate or the like and applying it to the portions or areas of theparts to be joined. Vanadium hydride can be substituted for the powderedelement vanadium with equally favorable results.

The alloy could also be made into a thin foil and the foil then appliedas strips on the surfaces to be joined. This form is preferred whenaccurate control of the amounts is desired.

In accordance with the invention, the alloy is used to join togetherrefractory metals such as molybdenum or tungsten. The term refractorymetals is defined as the group consisting of hafnium, zirconium,vanadium, niobium, tantalum, molybdenum, and tungsten.

After the alloy mixture has been applied to the areas of the parts to bejoined the materials are heated to above 1,650C in hydrogen, vacuum, orother nonreacting atmosphere. The tungsten vanadium powder forms aliquid phase and flows and wets the surfaces of the refractory metalsbeing joined together.

Referring now to FIG. 1, an example of the invention is illustrated. Ahollow cathode 2 is illustrated comprising a barium-strontium-tungstatematrix 4 within a molybdenum metal sleeve 6. In the illustratedembodiment, the brazing alloy is used at 30 to join cathode 2 to aheater 10 both for mechanical strength and to increase the heatconductivity from heater 10 to cathode 2. Heater 10 comprises a circularring 12 of a refractory metal such as molybdenum having embedded thereinheater coils 20 which comprise tungsten wire wound around aMolybdenum-Rehnium core. The heater coils heat the molybdenum metal andthe heat is then transmitted to the cathode through braze 30.

To join the cathode and the heater together, a powdered mixture of thealloy of the brazing material of the invention is evenly spread on thesurface of the heater and the cathode then placed thereon. This subassembly is then put into a reducing furnace and heated to a temperatureslightly above 1,650C until the alloy was visually seen to flowindicating that the alloy had become liquid to wet and weld theadjoining surfaces. The welded subassembly was then removed from the hotzone to avoid over exposure of the cathode emitting materials to thealloying temperature.

Tubes fabricated using such cathode-heater subassemblies brazed togetherwith brazing material of the invention have been found to be of superiorperformance over an extended life period considerably longer thansimilar tubes fabricated using prior art brazing materials.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

weight percent vanadium.

3. The assembly of claim 1 wherein said alloy consists essentially oftungsten and vanadium in a ratio of 4.5 atomic percent tungsten to about95.5 atomic percent vanadium.

4. The assembly of claim 1 wherein the ratio of tungsten and vanadium inthe alloy comprises a eutectic mixture.

2. The assembly of claim 1 wherein the brazing alloy comprises about 15weight percent tungsten and about 85 weight percent vanadium.
 3. Theassembly of claim 1 wherein said alloy consists essentially of tungstenAnd vanadium in a ratio of 4.5 atomic percent tungsten to about 95.5atomic percent vanadium.
 4. The assembly of claim 1 wherein the ratio oftungsten and vanadium in the alloy comprises a eutectic mixture.